Select Title,Count(1) as [Count]
From
(
SELECT title FROM books_north bn
union all
select title from books_south bs
) A
Group by Title

Another approach using FULL OUTER JOIN (If your RDBMS supports)

SELECT COALESCE(bn.Title, bs.title) as title,
( bn.[count] + bs.[count] ) AS [Count]
FROM (SELECT title,
Count(1) AS [count]
FROM books_north
GROUP BY title) bn
FULL OUTER JOIN (SELECT title,
Count(1) AS [count]
FROM books_south
GROUP BY title) bs
ON bn.Title = bs.Title

Regarding your error message, Title column is present in the both the table so when you select the Title column you need to tell the compiler from which table you want to select Title column. It can be done by giving a alias name to the tables in Join

Email codedump link for What am I missing this? Do I need to use JOIN or UNION or a subquery?